1. Technical Field of the Invention
The present invention relates to an optical pickup and an optical disk apparatus, applicable, for example, to an optical disk apparatus adapted to access a high-density recorded optical disk. In the present invention, an optical path length difference generator means is disposed in an optical path to produce an optical path length difference between two luminous fluxes obtained through resolution of return light, hence realizing a simplified structure, which is capable of preventing any characteristic deterioration of a focus error signal that may otherwise be caused by horizontal deviation.
2. Description of Prior Art
In any conventional optical disk apparatus known heretofore, it has been customary that focus control of a laser beam to be irradiated to an optical disk is executed on the basis of a focus error signal of a level changed in accordance with the amount of a focus error. Detection of such a focus error signal is performed by the use of an astigmatism method, Foucault method, SSD (Spot Size Detection) method or the like.
Out of these detection methods mentioned above, when the astigmatism method is applied to an optical disk where land-groove recording is adopted, a positional deviation is detected of a xe2x80x9cjust focusxe2x80x9d position between a land portion and a groove portion. Therefore, in regard to any optical disk adopting such land-groove recording, principally the Foucault method or SSD (Spot Size Detection) method is applied.
FIG. 11(A) is a schematic diagram showing an exemplary optical pickup in which the Foucault method is applied. In this optical pickup 1, a semiconductor laser 2 emits a laser beam L1 therefrom, and a collimator lens 3 converts the laser beam L1 into substantially parallel light rays. A beam splitter 4 reflects the incoming laser beam L1, which is incident thereon from the collimator lens 3, toward an optical disk 5, and then an objective lens 6 condenses the laser beam, which is obtained from the beam splitter 4, onto an information recording plane surface of the optical disk 5.
As a result, return light L2 is obtained from the optical disk 5. Then, this return light L2 is incident upon the beam splitter 4 by way of the optical path of the laser beam L1 in the reverse direction. The beam splitter 4 transmits the return light L2 therethrough to separate the optical path of the laser beam L1 from that of the return light L2. A collimator lens 7 converts the return light, which is emitted from the beam splitter 4, into a converged luminous flux, and then a half mirror 8 splits the return light, which has been converted into such a converged luminous flux, into two luminous fluxes.
A light sensor 9 receives and senses the return light reflected by the half mirror 8. In the optical disk apparatus, the sensed result of the light received by the light sensor 9 is processed through current-to-voltage conversion to thereby generate a reproduced signal RF whose level is changed in accordance with pit trains or the like formed on the optical disk 5. The reproduced signal RF thus obtained is processed to reproduce the data recorded on the optical disk 5.
In a case in which the Foucault method is employed, a Foucault prism 10 is disposed in the optical path of the return light converted into a converged luminous flux as mentioned. The Foucault prism 10 is so shaped that its center protrudes, hence resolving the return light into two luminous fluxes respectively having outgoing directions that are inclined obliquely to the optical axis. In this optical pickup 1, the return light is resolved substantially symmetrically with respect to the optical axis of the return light, and the luminous fluxes mutually intersect in the outgoing directions.
In the optical pickup 1, the return light L2 emitted from the Foucault prism 10 is received by a predetermined light sensor 11. When the light receiving plane of the light sensor 11 and the information recording plane surface of the optical disk 5 are held in a conjugate relation, as shown in FIG. 11(C), the two luminous fluxes form respective focal points on the light receiving plane of the light sensor 11. Consequently, two beam spots SP1 and SP2 are formed by the two luminous fluxes (hereinafter this state will be referred to as a xe2x80x9cjust focus statexe2x80x9d).
When an objective lens 6 is moved toward the information recording plane of the optical disk 5 and the emission point of the return light recedes equivalently from the objective lens 6, the two beam spots SP1 and SP2 formed on the light receiving plane are positionally changed in a manner to approach the optical axis. Also, the shapes thereof are enlarged as shown in FIG. 11(B), since the outgoing directions of such two beam spots are so inclined as to mutually intersect by the Foucault prism 10.
To the contrary, when the objective lens 6 is moved away from the information recording plane of the optical disk 5 and the emission point of the return light equivalently approaches the objective lens 6, the two beam spots SP1 and SP2 formed on the light receiving plane are positionally changed in a manner to recede from the optical axis and also the shapes thereof are enlarged as shown in FIG. 11(D).
Utilizing such relationship, there are formed, as shown in FIGS. 11(A), 11(B), and 11(C), in the light sensor 11, light receiving planes 11A and 11B defined by respectively dividing the light receiving plane into two areas a, b and c, d in directions where the respective focal points of the beam spots SP1 and SP2 are changed with reference to the return-light focal point in the just focus state. In the optical disk apparatus, the sensed results of the received light in such areas a to d are processed through current-to-voltage conversion, and the results of the current-to-voltage conversion are represented by codes which correspond respectively to the areas a to d, thereby generating a focus error signal FE expressed by an arithmetic equation of
FE=(a+d)xe2x88x92(b+c).
Further, the objective lens 6 is so moved as to reduce the level of the focus error signal FE to zero as indicated by an arrow A, hence executing focus control.
FIG. 12 is a perspective view showing principal portions of a light accumulator applied to an optical pickup that is based on the SSD method. In this optical pickup, a laser beam L1 emitted from the light accumulator 15 is condensed on an optical disk by means of an objective lens, and return light L2 obtained from the optical disk is received via the objective lens and then is introduced to the light accumulator 15.
The light accumulator 15 reflects the laser beam L1, which is emitted from a semiconductor laser diode chip 17, onto an inclined surface 16A of a prism 16 produced by cutting a glass material, and then sends the laser beam L1 toward the objective lens.
The light accumulator 15 introduces the return light L2, which is reflected by way of the optical path of the laser beam L1 in the reverse direction, from the inclined surface 16A into the prism 16, and then separates the return light L2 into transmitted light and reflected light by the lower plane of the prism 16. The light accumulator 15 of FIG. 12 enables a light sensor 18 to receive the transmitted light obtained through the lower plane of the prism 16. The light accumulator 15 further reflects the reflected light from the lower plane of the prism 16 by the upper plane thereof and, after transmitting the reflected light through the lower plane, enables the light sensor 19 to receive the reflected light. Thus, the optical pickup using this light accumulator 15 is so structured that the diameters of beam spots formed on the respective light receiving planes of the light sensors 18 and 19 are substantially equalized to each other in a just focus state. If the distance to the optical disk is changed from the just focus position, the diameters of the beam spots formed on the respective light receiving planes of the light sensors 18 and 19 are changed complementarily in accordance with the direction of such change.
The light sensors 18 and 19 are constituted integrally on a single semiconductor substrate. In the light sensors 18 and 19, the light receiving planes are so divided as to be capable of detecting the shapes of return-light beam spots formed on the respective light receiving planes. In the optical disk apparatus employing such optical pickup, the sensed results of the received light detected on the light receiving planes of the light sensors 18 and 19 are processed through current-to-voltage conversion, and then the results of such current-to-voltage conversion are calculated to generate a focus error signal in conformity with the diameters of the beam spots formed respectively on the light sensors 18 and 19.
In comparison with the above example, FIG. 13 is a perspective view showing another light accumulator 21 applied to an optical pickup based similarly on the SSD method. In this optical pickup, the return-light optical system subsequent to the collimator lens 7 described in regard to FIG. 11 is replaced with the light accumulator 21.
The light accumulator 21 of FIG. 13 comprises a composite prism 24 which consists of a right-angled triangular prism 22 and a parallelogrammic prism 23 (i.e., a prism having a parallelogram shape) adhered to an oblique surface of the right-angled triangular prism 22, wherein the composite prism 24 is disposed on light sensors 25 and 26. These light sensors 25 and 26 are constituted integrally on a single semiconductor substrate. Similarly to the light sensors 18 and 19 described above in regard to FIG. 12, the light receiving planes are so divided as to be capable of detecting the diameters of beam spots formed respectively on the light receiving planes.
The light accumulator 21 of FIG. 13 inputs return light L2 from the composite prism 24, and then resolves the return light L2 into transmitted light and reflected light by the joined surfaces of the right-angled triangular prism 22 and the parallelogrammic prism 23. The light accumulator 21 receives, by the light sensor 25, the transmitted light obtained through the lower surface of the composite prism 24, while it receives, by the light sensor 26, the reflected light obtained from the inclined plane and transmitted through the lower surface of the composite prism 24.
In the optical disk apparatus employing such optical pickup, the sensed results of the received light detected by the light sensors 25 and 26 are processed in the same manner as in the aforementioned optical disk apparatus described in regard to FIG. 12, whereby a focus error signal is generated.
Such prior efforts had experienced difficulties. In the light sensor 11 based on the Foucault method, there is formed a dead region AR (FIG. 11), which is a slit-shaped zone completely unused for reception of any return light, between the areas a and b, and also between the areas c and d defined by dividing the light receiving planes 11A and 11B.
When a just focus state is kept in the optical disk apparatus based on the Foucault method, the entire amount of the return light is condensed onto the dead region AR, so that it becomes necessary to prepare an exclusive light sensor for detection of a reproduced signal RF. Consequently, when the Foucault method is adopted, there arises a problem that the optical pickup is structurally complicated.
If the light sensor 11 is horizontally deviated, as shown in FIG. 14(A), in the direction of the array of its light receiving planes 11A and 11B, then the return light is condensed onto only one of the divided areas even in a just focus state. In this condition, as compared with another case of FIG. 14(B) without any positional deviation, there is formed a dead region, where the level of a focus error signal FE is not changed at all, in the vicinity of the just focus point, as shown in FIG. 14(C). Thus, in case the Foucault method is adopted, another problem is existent in that the focus error signal characteristic is extremely deteriorated due to such a horizontal positional deviation to eventually bring about a disadvantage of requiring a time for adjustment to attain a positional coincidence.
For solution of the above problems, there may be contrived a mode of employing the SSD method described in connection with FIGS. 12 and 13. However, it is difficult in such a mode to apply a three-spot tracking control process and is impossible to flexibly comply with any structural change.
The present invention has been accomplished in view of the points mentioned above. It is thus an overall object of the invention to provide an optical pickup of a simplified structure that is capable of preventing, even in focus control by the Foucault method, any characteristic deterioration of a focus error signal derived from a horizontal positional deviation. Another object of the invention resides in providing an optical disk apparatus where such an optical pickup is used.
For the purpose of solving the problems mentioned, the invention, when applied to an optical pickup or an optical disk apparatus, an optical path length difference generator means for rendering different the lengths of the optical paths of two luminous fluxes obtained by resolving return light.
According to the structure of the invention, an optical path length difference generator means is provided for rendering different the lengths of the optical paths of two luminous fluxes obtained by resolving return light, so that even when one luminous flux forms a focal point on the light receiving plane, the other luminous flux is condensed, on the light receiving plane, in the shape of a large beam spot anterior or posterior to the focal point.
Therefore, if the configuration is so arranged as to generate a focus error signal by dividing the light receiving plane in a manner to detect the diameters of beam spots, it is still possible to avoid an undesired situation where the entire amount of the return light is condensed on a dead region of a light sensor, and thus a reproduced signal can be obtained from the sensed results of the received light used for generation of the focus error signal. Consequently, the whole structure can be simplified correspondingly thereto.
When the lengths of the optical paths are thus rendered different from each other, if focus control is so executed as to equalize the diameters of beam spots formed by two luminous fluxes on the light receiving planes, then one luminous flux is condensed anterior to the focal point while the other luminous flux is condensed posterior to the focal point. Accordingly, in comparison with a known case where respective focal points are formed, focus control can be so executed as to attain a control target state with large beam diameters, hence achieving effective avoidance of any sharp characteristic change of the focus error signal derived from the horizontal positional deviation.